From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 20 Feb 2008 08:32:45 +0100 From: Willy Tarreau To: Andi Kleen Subject: Re: [PATCH 1/3] Fix Unlikely(x) == y Message-ID: <20080220073245.GA28524@1wt.eu> References: <47B70A61.9030306@tiscali.nl> <20080216094226.1e8eede1@laptopd505.fenrus.org> <200802191333.53607.nickpiggin@yahoo.com.au> <20080219055806.GA8404@1wt.eu> <20080219092846.GB6485@one.firstfloor.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20080219092846.GB6485@one.firstfloor.org> Cc: Nick Piggin , Roel Kluin <12o3l@tiscali.nl>, lkml , linuxppc-dev@ozlabs.org, cbe-oss-dev@ozlabs.org, Arjan van de Ven List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tue, Feb 19, 2008 at 10:28:46AM +0100, Andi Kleen wrote: > > Sometimes, for performance critical paths, I would like gcc to be dumb and > > follow *my* code and not its hard-coded probabilities. > > If you really want that, simple: just disable optimization @) already tried. It fixed some difficulties, but create new expected issues with data being reloaded often from memory instead of being passed along a few registers. Don't forget that optimizing for x86 requires a lot of smartness from the compiler because of the very small number of registers! > > Maybe one thing we would need would be the ability to assign probabilities > > to each branch based on what we expect, so that gcc could build a better > > tree keeping most frequently used code tight. > > Just use profile feedback then for user space. I don't think it's a good > idea for kernel code though because it leads to unreproducible binaries > which would wreck the development model. I never found this to be practically usable in fact, because you have to use it on the *exact* same source. End of game for cross-compilation. It would be good to be able to use a few pragmas in the code to say "hey, I want this block optimized like this". This is what I understood the __builtin_expect() was for, except that it tends to throw unpredicted branches too far away. > > Hmm I've just noticed -fno-guess-branch-probability in the man, I never tried > > it. > > Or -fno-reorder-blocks Thanks for the hint, I will try it. Willy